The lifetime of most implantable electro-devices is limited by the longevity of the power source. As the need for autonomous implanted sensors and devices increases, this will become even more critical. Future advances in power source design must continue to accommodate a range of battery functions in implants. In pacemakers, half the battery capacity is used for cardiostimulation and the other half to gather and telemeter information for monitoring purposes. Defibrillator and neurostimulator activities require burst mode applications of electrical shock that deplete the energy stored in the battery more quickly. Pacemakers and defibrillators are currently powered by primary batteries. Neurostimulators use primary or secondary batteries, the latter recharged by conversion of radio-frequency electromagnetic energy to electrical energy. Simpler and more effective recharging techniques are needed to power these types of devices.
Thus, a need exists for the development of power sources for implanted electro-medical devices. For example, when the battery of an implanted pacemaker has discharged to a predetermined threshold, surgery must be scheduled to replace the entire implant. Moreover, despite advances in battery technology, batteries currently consume 50% or more of the weight and volume of most implanted devices. Many new and emerging medical device technologies, such as enhanced inter-device telemetry, automated wireless alarm signaling, advanced sensors, and infusion pump therapies, continue to place demands on powering implanted systems .
The global market for implanted medical devices is significant and growing. Over 600,000 pacemakers were implanted worldwide in 2003, with 3 million of the devices in use at that time. In 2004, the overall market for cardiac rhythm management was estimated to be $8.9 billion, and by 2007 the total market for implantable and ingestible devices was predicted to exceed $24.4 billion. In addition to pacemakers and defibrillators, implantable devices now include pumps for diabetes and pain management, neurostimulators for pain therapy, and devices similar to pacemakers to electrically stimulate the stomach, throat, and other muscles.
Rechargeable batteries heretofore have not found wide use in implantable medical device applications. Patient compliance and recharge frequency limited the utility of rechargeable batteries. However, the increasing power needs for implanted devices directed at new therapies (defibrillators, drug pumps, left ventricular assist devices, and neurostimulators) combined with the development of advanced rechargeable lithium ion chemistries have awakened new interest in the use of rechargeable batteries in implants. A new recharging technology which is simple and user friendly, coupled with the improvements in rechargeable lithium ion or more advanced batteries, will prompt the medical community to rethink the issue of primary versus secondary batteries in conjunction with implantable medical devices.
The present method of recharging implantable batteries has been the radio frequency (RF) induction technique. Two coils, one outside the body and the second inside and connected to the device are placed in close proximity. A current generated in the outer primary coil induces a current in the inner secondary coil, and the voltage so generated is used to recharge the battery. While this technique can provide considerable power transmission, it has its disadvantages. The more power required, the larger the coils, leading to a heavy device with a large footprint both inside and outside of the body. Orientation of the two coils is critical, there is the potential for electromagnetic interference with other devices in the area, and nearby metal can be heated by eddy currents. Dielectric attenuation restricts usage to transmission through a few millimeters of skin. The ferrous metal parts inhibit magnetic-resonance imaging.
Several workers have proposed various implantable devices configured for transporting acoustic waves transmitted through the tissue of a patient to electrical current for powering the implanted device. See for example U.S. Pat. No. 7,283,874 to Penner. See also U.S. Pat. Nos. 5,749,909 to Schroeppel et al. and 6,185,452 to Schulman et al. Devices such as described above have, in practice, not proved to be satisfactory in that they were extremely limited in the amount of power that could be transmitted through the skin and tissue of the patient without causing excessive heating with the potential for tissue damage and the like, or because their objectives required lower power. This limitation of the prior art is, in part, a result of the use of PZT-based piezoelectric ceramic materials which are relatively inefficient when it comes to converting energy and transferring the energy through the skin and tissue.